A nitrile rubber represented by an acrylonitrile-butadiene copolymer rubber (NBR) has hitherto been widely used as an oil-resistant rubber material in various fields. A highly saturated nitrile rubber prepared by reducing the amount of carbon-carbon unsaturated bond in the backbone chain was developed for satisfying a demand of placing on the market a rubber vulcanizate having enhanced heat-aging resistance and high tensile strength.
Hydrogenated NBR has a structure such that butadiene units in the molecular chain have been hydrogenated and thus the content of unsaturated bonds, i.e., double bonds, has been reduced to zero or an extremely small value. Therefore hydrogenated NBR has a remarkably enhanced resistance to oxidative degradation and Is highly evaluated as a heat aging-resistant rubber.
However, the currently available highly saturated nitrile rubber still has unsaturated bonds even only in a minor amount, the heat aging resistance in a hot-air atmosphere is liable to be poor. Therefore, additives to be incorporated in the highly saturated nitrile rubber for enhancing the heat aging resistance are being developed.
For example, Japanese Unexamined Patent Publication No. H11-293039 proposed the incorporation of a strong base, a salt of a strong base with a weak acid, or a salt of a weak acid in highly saturated nitrile rubber to minimize the reduction of elongation at break as encountered when the highly saturated nitrile rubber is kept in a hot-air atmosphere.
The incorporation of the proposed ingredients can reduce the reduction of elongation at break, but occasionally, the reduction of elongation cannot be minimized to a desired level. There is still a demand for the enhancement of heat aging resistance.